Memory devices can include internal, semiconductor, integrated circuits in computers or other electronic devices. There are many different types of memory including random-access memory (RAM), read only memory (ROM), dynamic random access memory (DRAM), static RAM (SRAM), synchronous dynamic RAM (SDRAM), and flash memory.
Flash memory devices have developed into a popular source of non-volatile memory for a wide range of electronic applications. Flash memory devices typically use a one-transistor memory cell that allows for high memory densities, high reliability, and low power consumption. Common uses for flash memory include personal computers, personal digital assistants (PDAs), digital cameras, and cellular telephones. Program code and system data such as a basic input/output system (BIOS) are typically stored in flash memory devices for use in personal computer systems.
As the performance and complexity of electronic systems increase, the requirement for additional memory in a system also increases. However, in order to continue to reduce the costs of the system, the parts count must be kept to a minimum. This can be accomplished by increasing the memory density of an integrated circuit.
Memory density in a non-volatile memory can be increased by using multiple level cells (MLC). MLC memory can increase the amount of data stored in an integrated circuit without adding additional cells and/or increasing the size of the die. The MLC method stores two or more data bits in each memory cell.
MLC, however, requires tighter control of the threshold voltages in order to use multiple states per cell. An MLC memory device typically has a higher bit error rate than a single level cell (SLC) memory device due, in part, to the increased quantity of states requiring more closely spaced threshold voltages. A bad bit in a memory device used to store photographs can be tolerated more easily than a bad bit in a memory device that stores code. A bad bit in a photograph might only produce a bad pixel out of millions of pixels while a bad bit in code or other data could mean a corrupted instruction that affects the operation of an entire program.
For the reasons stated above, and for other reasons stated below that will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a memory device having higher density with increased reliability.